Regenerative Braking - D or B ?

I think you nailed it

 

In 'B' mode, if it's the same as my gen4 2017, it uses the engine's compression to do "engine braking". Unless I misread the documentation, it's basically to prevent the brakes from overheating while on a long descent. I don't know if the engine is actually firing or if it runs dry in that mode. If it's firing, then it's less economical than simply Drive mode for normal driving. It also puts more wear on the engine.

 

You mostly got it there.

You always get regen. B mode is just a way of hinting the computer that your current driving situation involves more available kinetic energy than usual; it can then plan to charge the battery over a longer period of time and also dump energy by spinning the engine as a dead load.

The result is more drag for safer hill descent and smarter battery management.

 

I couldn't say, to be honest. Your model is a bit different from ours.

I will say that Toyota designed them all such that (on flat land) an experienced right foot can gain the maximum recovery possible without ever shifting out of D.

Most of the trick is to make the act of 'braking' take as long as possible, to spread that energy surge across time. So brake early, stay light on the pedal, and ease your way up to the stop point for best results.

 

N/M

 

I use B in 2020 a lot. Just to keep foot off brake pedal down hills more and coming to a stop. I feel it may be advantageous to charge, but don’t know why. Foot on brake does the same thing as far as I can tell.

 

With the Prime in EV mode B is more regen from MG1 and MG2 than the regen in D (since both MG1 and 2 can be either motor of generator when one of the ECU's calls either or both of them to do work, (don't ask me to explain that any further, cause I can't)). And slightly more aggressive speed reduction than D without using the brake pedal.
In HV mode (and the engine running) B mode spins up the engine to a higher RPM than D mode would normally spin the engine and that specific RPM. B might also provide more regen from MG1 and / or MG2 than D normally would, and the engine might also be only pumping air with no fuel, but I don't have techstream and I haven't found a way yet to confirm if B modes regen with engine on is cutting fuel and only pumping air like has been described for earlier Gen regular Prius and / or If B calls MG1 and / or MG2 for more regen than D modes normal regen with engine spinning.
If the engine is Off while in HV mode and the driver selects B the engine will start and spin up, except if under 20mph, where the engine will stay off under most conditions and use only MG1 and / or MG2 for more regen than D and slightly more aggressive speed reduction than D without using the brake pedal.

If the Prime behaves for you differently than I've described it behaving for me, that would most like be because your driving patterns differ from mine at the specific combinations of sensor set points, of which there are more than I'll ever figure out.

 

That's the difference in a word.

There's a 9%er on 3/4 mile between my house and my nearest relative's, so we make fair use of B mode in the Prius or 2nd/3rd (of 6) in the other car.

This lil Prius c is a billygoat up on these little East-coast mountain roads. Fun driving!

 

Just depends on whether you want to slow that hard all the time.

New owners often get this idea that more regen charge into the battery is advantageous. It isn't; every joule of energy recovered and shoved into the battery is taxed once by conversion losses on the way in, and again later by conversion losses on the way back out.

Electrical regen is better described as the worst thing you can do with your kinetic energy, except for burning it off in the friction brakes or the engine, which are the only things it's less wasteful than.

The better you can learn to drive so that none of those things happen any more than necessary, the better your results will be.

 

Usually my driving is 100% ev around town. There is a semi circle display that I leave it there all the time. The blue charge bar has this dotted line at the bottom I assume is some kind of limit to charge output, but the blue bar can go below the dotted line. Or dashed whatever it is. So I have no experience with B turning on the engine in the Prime. Once in awhile I get to use it in hybrid mode, but I don’t use B then. I barely use anything in the car, no music, no nothing. Daughter says how can you drive around like that in silence?
I didn’t realize it didn’t have a cd player, how can I listen to music if I do want to? I have some music cd’s.

 

It's cool, isn't it?

And yet, what you just did with that 1%-of-battery worth of energy was in fact pretty wasteful. Only burning it off with the friction brakes or the engine would have been worse. Whenever you can manage the driving so you avoid slowing like that, it's even better. Don't "reclaim" the energy when you can keep it in the form of motion.

 

So, with the help of Hybrid Assistant today, I was able to see how Regen works in my 2017 Prime. In B mode, as soon as I let go the accelerator, it applied a max of 20 KW Regen. In D mode, it applied a max of 7 KW Regen, but once I started to press on the brake pedal, Regen went up to 30 KW. May be B mode can get it up to that level but I haven't seen it. It's nice to see D mode modulates Regen based on how hard you press on the brakes.

One thing I miss is a Hold mode, where the car applies the physical brakes once stopped so you can let go the brake pedal.

 

It seems to have a cut off for the electric motor when the brake is applied hard enough. It’s much more efficient than a torque converter slipping of course. I think the Prius is about perfect at and from a stop, although must keep the foot on the brake. Have to try it how it holds on a steeper hill soon.
I go down a long steep hill at least once a day. I can regen quite a lot as I have to stop in the middle on the way down. Even in D it has a good amount of regen. Better than no regen, on the way back it consumes about twice or more what was regenerated going down. Without regen the end result would be 30-40% more energy used for the round trip, as a rough guess.

 

Are you sure it's missing? My 2010 has it.

You press the brake pedal however hard you have to for the car to stop, and then without releasing it, in a second motion you push it down a little farther. There's a beep and the traction light starts flashing, and then if you want you can release the pedal, and the brakes will hold for up to a couple seconds, or until you are pressing the go pedal.

 

Completely automatic in our 2018 c. If you stop on an upgrade, the mechanical brakes hold the car until you're into the throttle. I've never even noticed if there's a control to override or otherwise influence the thing.

 

Thanks, I'll give this a try. I don't recall seeing this in the manual though.